1. Field of the Invention
The present invention generally relates to a magnetic recording and/or reproducing apparatus of a type having a so-called double-level recording or triple-level recording capability and, more particularly, to a tracking control system for the magnetic recording and/or reproducing apparatus.
The magnetic tape player double-level recording model is well known as the type wherein video and audio information are recorded on outer and inner strata of a length of magnetic tape, respectively. In reproducing the video and audio information recorded on the outer and inner strata of the length of magnetic tape, at least one head assembly comprised of a video pick-up head and an audio pick-up head, positioned on the trailing side of the video pick-up head with respect to the direction of scan, is utilized to scan the length of magnetic tape with the video and audio pick-up heads picking up the video and audio information, respectively. This technique is widely used in a Hi-Fi video tape player operable according to the VHS standards.
On the other hand, examples of an automatic tracking control system used in the currently commercially available magnetic tape player of a single-level recording model are disclosed in the Japanese Patent Publications examined Nos. 55-51256 and 55-51257, the Japanese Laid-open Patent Publications Nos. 57-120226, 58-1843, 58-154985, 60-163224 and 61-29282. Among them, Japanese Patent Publications examined Nos. 55-51256 and 55-51257 are most relevant to the present invention and a schematic block circuit diagram thereof is reproduced in FIG. 1 of the accompanying drawings. This prior art automatic tracking control system shown in FIG. 1 will now be discussed with the aid of FIGS. 2 and 3 which illustrate a phase versus envelope voltage characteristic and a tracking phase versus envelope voltage characteristic, respectively, both exhibited by the prior art automatic tracking control system.
Referring first to FIG. 1, a length of magnetic tape 1 has video information 2 and control signals 3 both recorded thereon. The video information 2 so recorded is comprised of recorded tracks extending parallel to each other and slantwise relative to the longitudinal axis of the length of magnetic tape 1, whereas the control signal 3 utilizable for the tracking servo control are recorded in spaced relation with each other and along one of the opposite side edges of the length of magnetic tape 1.
A rotary drum 6 supported for rotation about the center thereof and adapted to be driven in one direction by a drum drive motor 8 controlled by a drum motor control circuit 7 carries rotary magnetic video heads 4a and 4b in spaced relation to each other for rotation together with the rotary drum 6 at a predetermined peripheral velocity for sequentially picking up video signals recorded on the tracks during the scanning operation in which the rotary drum 6 is driven at high speed while the length of magnetic tape 1 is transported in one predetermined direction, shown by the arrow 15, from a tape supply reel towards a tape take-up reel. The control signals 3 are, during the movement of the length of magnetic tape 1, successively picked up by a stationary control head 5 fixedly supported so as to confront the edge of the length of magnetic tape 1 where the control signals 3 are recorded.
During the scanning operation, the length of magnetic tape 1 is transported in the direction of the arrow 15 by a capstan 14 drivingly coupled with a capstan drive motor 9 through a drive pulley 12 and an endless belt 13, said capstan drive motor 9 having a frequency generator 10 capable of generating a frequency signal having a frequency proportional to the peripheral velocity of the rotary drum 6, which frequency signal is hereinafter referred to as a FG signal. The capstan drive motor (hereinafter referred to as CP motor) 9 is controlled by a capstan drive motor control circuit 11 in dependence on the FG signal generated from the frequency generator 10.
Each control signal 3 picked up by the stationary control head 5 is applied to a phase comparator 17 after having been amplified by an control amplifier 16. In addition to the amplified control signal, the phase comparator 17 receives through a phase adjusting circuit 20 a phase signal generated from a stationary detector head 19 operable to detect a magnetic flux emanating from a magnet piece 18 secured to the rotary drum 6 for rotation together therewith, said phase signal being indicative of the phase of revolution of the rotary drum 6.
A difference signal outputted from the phase comparator 17 is applied to the capstan drive motor control circuit 11 for controlling the capstan drive motor 9, then driven at about a predetermined speed by the control circuit 11, thereby to finely adjust the speed of movement of the length of magnetic tape 1 so that the phase of rotation of the rotary magnetic video heads 4a and 4b and the phase of the reproduced control signal 3 can have a predetermined phase relationship with each other determined by the phase adjusting circuit 20. Therefore, the rotary magnetic video heads 4a and 4b can successively scan recorded tracks 2 as allocated by the phase adjusting circuit 20.
On the other hand, a video FM signal reproduced by the rotary magnetic video heads 4a and 4b is, after having been extracted by a rotary transformer 21, amplified by a head amplifier 22 and is then supplied to an envelope detecting circuit 23 where the amplified video FM signal is subjected to an envelope detection, a detected signal from the envelope detecting circuit 23 being subsequently supplied to an integrating circuit 28 and also to a comparator 25. The output from the head amplifier 22 is also supplied to a peak hold circuit 24 operable to hold a maximum value of an envelope signal. An output from this peak hold circuit 24 and an output from the envelope detecting circuit 23 are supplied to the comparator 25 operable to compare the output voltage Vp of the output from the peak hold circuit 24 and the output voltage Ve of the output from the envelope detecting circuit 23 and to determine whether the difference between the output voltages Vp and Ve is higher than a threshold value eo or whether the difference in voltage is lower than the threshold value eo. An output from the comparator 25 is supplied to a differential circuit 26 which in turn outputs a positive or negative pulse signal each time the output from the comparator 25 reverses.
A flip-flop 27 interposed between the differential circuit 26 and the integrating circuit 28 is adapted to be triggered only when the differential circuit 26 applies the negative pulse signal, thereby to reverse the negative or positive output voltage level. An output from the flip-flop 27 is subsequently supplied to, and integrated by, the integrating circuit 28 which then provides an increasing or decreasing signal dependent on the voltage polarity of the output from the flip-flop 27, which increasing or decreasing signal is used to control the phase of the phase adjusting circuit 20.
With reference to FIG. 2, assuming that the phase of the phase adjusting circuit 20 is at a point a, the voltage of the output from the flip-flop 27 is of a positive voltage level, and the output from the integrating circuit 28 is being increased, the phase of the phase adjusting circuit 20 lies in a direction of increase towards a point b and then towards a point c. In correspondence therewith, the output from the envelope detecting circuit 23 increases progressively to increase the envelope voltage Vp to a maximum value Vpmax and then decreases to decrease the envelope voltage Vp down to a value lower than the maximum value Vpmax. When the phase of the phase adjusting circuit 20 subsequently attains a point d, the difference between the envelope voltage Ve of the envelope detecting circuit 23 and the maximum value Vpmax of the hold voltage Vp of the peak-hold circuit 24 becomes equal to the predetermined threshold value eo and, therefore, the comparator 25 reverses its output level from positive to negative, with the consequence that the differential circuit 26 generates the negative pulse causing the flip-flop 27 to be reversed to a negative voltage level. As a result thereof, the output from the integrating circuit 28 is caused to decrease and the phase of the phase adjusting circuit 20 is caused to decrease again towards the point c. The phase of the phase adjusting circuit 20 shifts from the point b to the point d, and the output voltage Ve of the envelope detecting circuit 23 is controlled to shift between the values Vpmax and eo in a direction shown by the arrows. Thus, if the threshold value eo is carefully selected to an appropriate value, the tracking control hitherto performed manually can be automatically performed.
Since the prior art automatic tracking control system is so constructed as hereinabove discussed, the envelope voltage L obtained by varying the tracking phase as shown in FIG. 3 does, in the case of, for example, an SP mode exhibiting a characteristic shown by a curve A in FIG. 3, vary from a value L0 to a value L1 as the tracking phase increases from t0 to t1, and remains without changing when the voltage thereof increases. When the tracking phase varies from t2 to t3, the level at which the value L3 becomes lower than the value L2 is detected, which value L2 is determined to be the maximum value and the tracking phase is set at t2.
Since when the envelope voltage falls it is immediately set to the maximum value as hereinabove described, it has been problematic that, where the envelope voltage exhibits a two-peak curve having a peak point at two locations, even though a higher peak is found in the right-hand direction, the tracking phase is set at a lower peak position on the left-hand side and, therefore, the tracking phase so obtained is not always appropriate.
In the case of an EP mode exhibiting a characteristic shown by a curve B in FIG. 3, since the tracking phase is varied until the maximum value of the envelope voltage is found, there is a problem in that it takes a relatively long time to converge at the maximum value and, moreover, where a flat region at the top of the characteristic curve is varying, the maximum value cannot be fixed, tending to fluctuate from an optimum value.
Because of the reasons discussed above, when the length of magnetic tape recorded in one magnetic tape player is to be reproduced with a different magnetic tape player, the best track position can not be assured since the track width of the video heads in the magnetic tape player used to reproduce differs from the recorded track width of the magnetic tape.
Where the magnetic tape player is of the double-level recording type such as a Hi-Fi video tape player according to the VHS scheme, wherein video and Hi-Fi audio information are recorded by different magnetic heads on outer and inner strata of a length of magnetic tape, respectively, and if the above described control system is employed only for the video signal, the tracking of the Hi-Fi audio signal will become insufficient to such an extent as to result in lowered signal-to-noise ratio, thereby posing a problem in that the magnetic tape player in which the above described control system is employed only for the video signal will no longer be utilizable. The reason therefor will now be discussed.
Before the reason is discussed, the principle of double-level recording employed in a Hi-Fi magnetic tape player will be described with reference to FIGS. 4(a) and 4(b).
As shown in FIG. 4(a), the video heads 4a and 4b mounted on the rotary drum 6 are spaced 180.degree. from each other about the axis of rotation of the drum 6 and, similarly, Hi-Fi audio heads 40a and 40b having a gap go of about 0.8 micrometer in width with respect to the direction of transport of the length of magnetic tape 15 as shown in FIG. 4(b) are mounted on the rotary drum 6 while spaced 180.degree. from each other about the axis of rotation of the drum 6, the video heads 4a and 4b being offset a predetermined distance (for example, 16 micrometers) relative to the audio heads 40a and 40b as shown in FIG. 5(b) in a direction parallel to the axis of rotation of the drum 6.
A relatively high recording current is allowed to flow across the gap of each audio head 40a and 40b to record Hi-Fi audio information on an inner stratum of a magnetic layer 1b of about 4 micrometers in thickness, formed on a length of base film 1a of about 16 micrometers in thickness. Subsequently, another recording current is allowed to flow across the gap g1 (0.3 micrometer in width) of each of the video heads 4a and 4b to record video information on an outer stratum, immediately above the above mentioned inner stratum, of the same magnetic layer 1b.
The manner in which the video and audio information are recorded on the length of magnetic tape 1 in the double-level fashion represents such a pattern as shown in FIG. 5(a) when viewed from the magnetic layer 1b of the length of magnetic tape 1. FIG. 5 applies where video and audio information recorded on the length of magnetic tape with the magnetic tape player employing the video and audio heads 4a, 4b and 40a, 40b arranged in the manner as shown in FIG. 4(a) is reproduced by the same magnetic tape player (this situation being hereinafter referred to as "self-recording and self-reproduction"), and tracking performed by such magnetic tape player is schematically illustrated in FIG. 5(c). As shown in FIG. 5(c), the tracking position at which the envelope LV of the video signal attains a maximum value and the tracking position at which the envelope LA of the Hi-Fi audio signal attains a maximum value lie at the same point t0 and, therefore, in order to control to the optimum tracking position, it suffices to determine the maximum value of the envelope of either one of the video signal and the audio signal. Accordingly, in the case of self-recording and self-reproduction, the automatic tracking control can be attained even with the prior art system whose principle has been described with reference to FIG. 2.
However, where the length of magnetic tape is reproduced by the magnetic tape player which is different from the magnetic tape player used to record such length of magnetic tape and wherein the video and audio heads are offset relative to each other at the different levels as shown in FIG. 6(a) (this situation being hereinafter referred to as "borrowed-recording and borrowed-reproduction"), the tracking position t1 at which the envelope LV of the video signal attains a maximum value and the tracking position t2 at which the envelope LA of the Hi-Fi audio signal attains a maximum value displaces relative to each other as shown in FIG. 6(c).
Problems similar to the above described problem can be found in the triple-level recording system wherein a pair of video heads and two pairs of audio heads are employed and arranged as shown in FIGS. 7(a) and 7(b) for recording, in a manner as shown in FIG. 7(b), a Hi-Fi audio signal, a PCM color signal and a luminance signal, all frequency-modulated in a manner as shown in FIG. 8.
More specifically, in the case of self-recording and self-reproduction as shown in FIG. 9, the respective positions at which the envelope LV of the luminance signal, the envelope LP of the PCM signal and the envelope LA of the Hi-Fi audio signal attain maximum values coincide at the point t0 as shown in FIG. 9(c) and, therefore, in order to control to the optimum tracking, it suffices to determine the maximum value of one of the luminance signal, the PCM color signal and the Hi-Fi audio signal.
However, in the case of borrowed-recording and borrowed-reproduction as shown in FIG. 10, the respective positions at which the envelope LV of the luminance signal, the envelope LP of the PCM color signal and the envelope LA of the Hi-Fi audio signal attain maximum values displace from each other as shown in FIG. 10(c).
In general, according to the VHS standards, the video track width w0 is fixed at 58 micrometers. However, the actually recorded video track width w1 is often found to be smaller than 58 micrometers, and the difference in level between the video heads and the Hi-Fi audio heads varies from one magnetic tape player to another, and, therefore, it is often found that the relationship among the tracking positions t0, t1, t2 and t3 correspondingly varies from one magnetic tape player to another.
According to the automatic tracking control system such as shown in and described with reference to FIG. 1, since the envelope signal is processed through both of the peak hold circuit and the detecting circuit and is then analog-processed in the later stages, the circuitry tends to be complicated and unstable with respect to change in ambient temperature. Moreover, since the circuitry is very sensitive to the envelope to such an extent that, although being controlled automatically, the tracking tends to shift slowly but assuredly, the prior art automatic tracking control system has not yet been employed in any commercially available magnetic tape players.